A process for making hydrogen with low to no CO2 production is disclosed in the present invention. It incorporates the concepts described in co-pending US patent application 2010-0037521, herein incorporated by reference, describes a process for making hydrogen by adjusting the conditions in the steam methane reformer (SMR) to produce more hydrogen and CO by converting more methane and subsequently converting more of the CO to Hydrogen in a lower temperature medium temperature shift or the combination of high temperature and low temperature shift reactors. This CO2 in the syngas is then removed by contacting with an amine wash and the hydrogen is purified in a pressure swing adsorption (PSA) unit—with the residue (tail gas) of the PSA being sent to the SMR furnace to provide the necessary fuel for the furnace. Supplemental fuel is provided typically by natural gas to provide the additional fuel needed to control the temperature of the SMR furnace. This process removes about 67% of the CO2 produced in the Hydrogen plant compared to a conventional steam methane reformer equipped with an amine contactor in which about 57% of the CO2 can be removed. The remaining CO2 is produced from remaining CO and Methane in the PSA tail gas and the supplemental natural gas fuel are combusted in the SMR furnace to CO2 and contribute the remaining CO2 which is not recovered and emitted in the Furnace flue gas. Co-pending US patent application 2010-0037521 further teaches that the CO2 recovery can be further increased to about 90% by increasing the SMR feed by 33% and reducing the hydrogen recovery in the PSA such that enough more hydrogen is passed to the tail gas and subsequently to the SMR furnace and no supplemental natural gas is supplied to the SMR Furnace.
Co-pending, as-yet unpublished patent application Ser. No. 12/970,041, herein incorporated by reference, teaches that the extent of pre-reforming can be increased by utilizing higher amounts of waste heat for pre-reforming. The reaction products from a first stage of pre-reforming is heated to a higher temperature by exchanging heat with flue gas or process gas and sent to a second adiabatic catalytic reactor in which the endothermic reforming reactions drop the temperature. The process can be repeated through up to 4 or 5 pre-reformers in series and subsequently increasing the amount of pre-reforming from about 8-10% per a single bed pre-reformer to up to 20-25%. With higher degree of pre-reforming, the firing duty of the main reformer is reduced.
Referring to Co-pending US patent application 2010-0037521, the inventors teach that CO2 emissions from an SMR can be reduced by reducing the amount of CO2 produced by burning hydrocarbons in the SMR furnace. Co-pending, as-yet unpublished patent application Ser. No. 12/970,041 teaches that by increasing the extent of pre-reforming utilizing waste heat as the heat source, that the firing duty of the main reformer is reduced. For example by using three stages of pre-reforming instead of one stage of pre-reforming, the CO2 emissions from a conventional SMR can be reduced by 5-6%. By utilizing the increased pre-reforming concepts disclosed in Ser. No. 12/970,041 in addition to the increased CO2 capture taught in invention 2010-0037521, the CO2 removed can be increased from about 67% to about 90% without lowering the PSA H2 recovery as taught in 2010-0037521. Another benefit of the invention is that by using waste heat from the SMR furnace to do additional pre-reforming, steam production is reduced and when combined with CO2 removal by an amine contactor, there is no net export of steam from the SMR.
CO2 recovery utilizing the present invention can be further increased to 100%. This is achieved by taking the flue gas from SMR furnace through a dryer to remove water and compressing it. Typical specification for Nitrogen used for Enhanced Oil Recovery is >95% nitrogen. The resulting flue gas from the present invention will contain >95% Nitrogen+Argon, <3.1% CO2 and less than 1.9% Oxygen and would be an excellent gas to be used for enhanced oil recovery. By utilizing the flue gas for Enhanced Oil Recovery, no flue gas is emitted from the SMR and therefore no CO2 or NOx emissions.
A preferred gas for enhanced oil recovery would contain very low oxygen content. To produce a flue gas with low oxygen content, the flue gas from the SMR is combined with purified hydrogen from the PSA and contacted over a bed of catalyst to promote combustion of H2 with O2 to form water. The resulting flue gas stream is dried to remove excess water and compressed and used for enhanced oil recovery. The composition of the flue gas stream would be >97% N2+Argon, <3% CO2 and <0.1% O2, <0.1% H2.
The production of hydrogen by the steam reforming of hydrocarbons is well known. In the basic process, a hydrocarbon, or a mixture of hydrocarbons, is initially treated to remove, or convert and then remove, trace contaminants, such as sulfur and olefins, which would adversely affect the reformer and the down stream water gas shift unit catalyst. Natural gas containing predominantly methane is a preferred starting material since it has a higher proportion of hydrogen than other hydrocarbons. However, light hydrocarbons or refinery off gases containing hydrocarbons, or refinery streams such as LPG, naphtha hydrocarbons or others readily available light feeds might be utilized as well.
The pretreated hydrocarbon feed stream is typically at a pressure of about 200 to 400 psig, and combined with high pressure steam, which is at a higher than the feed stream pressure, before entering the reformer furnace. The amount of steam added is much in excess of the stoichiometric amount. The reformer itself conventionally contains tubes packed with catalyst through which the steam/hydrocarbon mixture passes. An elevated temperature, e.g. about 1580° F., or 860° C., is maintained to drive the endothermic reaction.
Prereforming of hydrocarbons upstream of the SMR or ATR is a well known process. It converts heavier hydrocarbons (ethane and heavier) to methane. It may also convert some of the methane to hydrogen, CO, and CO2, depending upon the chemical equilibrium under the given conditions.
Prereformer utilizes waste heat in the flue gas or process stream, which otherwise may be utilized in raising steam. Utilization of high level heat (at about 1600° F. to about 900° F.) is thermodynamically more efficient when used for prereforming than for raising steam with boiling temperature of about 400° F. to 600° F. Disposal of excess steam is a problem in many plants.
Typically the feed (hydrocarbon and steam mixture) to the prereformer is preheated in the range of 850° F. to 1000° F. before contacting with a catalytic bed in an adiabatic reactor. The reactants come to a chemical equilibrium. The extent of conversion of methane to H2/CO/CO2 is a function of the reaction temperature, higher temperature favoring the conversion.
The inlet temperature of the feed to prereformer is limited by its potential to crack hydrocarbons and deposit carbon on the catalyst and the preheat coils. Heavier the feedstock, lower is the potential cracking temperature. For example, the feed temperature for typical light natural gas is limited to about 1000° F., while feed temperature for naphtha feed is limited to 850° F. The amount of waste-heat utilization for prereforming depends on the preheat temperature of feed mixture. There is a need for a process that can utilize larger amounts of waste heat for prereforming.
The effluent from the reformer furnace is principally hydrogen, carbon monoxide, carbon dioxide, water vapor, and methane in proportion close to equilibrium amounts at the furnace temperature and pressure. The effluent is conventionally introduced into a one- or two-stage water gas shift reactor to form additional hydrogen and carbon dioxide. The shift reactor converts the carbon monoxide to carbon dioxide by reaction with water vapor, which generates additional Hydrogen. This reaction is endothermic. The combination of steam reformer and water gas shift converter is well known to those of ordinary skill in the art.
If CO2 capture from the high pressure syngas stream exiting the water gas shift unit is desired, the shift converter effluent, which comprises hydrogen, carbon dioxide and water with minor quantities of methane and carbon monoxide is introduced into a conventional absorption unit for carbon dioxide removal. Such a unit operates on the well-known amine wash or other solvent processes wherein carbon dioxide is removed from the effluent by dissolution in an absorbent solution, i.e. an amine solution or potassium carbonate solution, respectively. Conventionally, such units can remove up to 99 percent or higher of the carbon dioxide in the shift converter effluent.
The effluent from the carbon dioxide absorption unit is introduced into a pressure swing adsorption (PSA) unit. PSA is a well-known process for separating essentially pure hydrogen from the mixture of gases as a result of the difference in the degree of adsorption among them on a particulate adsorbent retained in a stationary bed.
Conventionally, the remainder of the PSA unit feed components, after recovery of pure hydrogen product, which comprises carbon monoxide, the hydrocarbon, i.e. methane, hydrogen and carbon dioxide, is returned to the steam reformer furnace and combusted to obtain energy for use therein
To practice CO2 emissions capture from such hydrogen plants, one must consider total emissions resulting from the plant, which includes CO2 recovery from reformer furnace flue gas as well.